It is well known that liquid crystalline polymers can be made into fibers having high strength and high modulus. Generally, the tensile properties of asspun fibers made from liquid crystalline polymers are improved significantly if the fibers are heated to elevated temperatures in an unstressed state for a period of hours. For comparison, semi-crystalline polymers that are not liquid crystalline, such as poly(ethylene terephthalate), develop their best properties when they are drawn to &gt;100% of their original length by applying a stress at elevated temperatures.
There are a few reports of improvements in tensile properties of liquid crystalline polymers that are achieved by application of stress at elevated temperature. Thus, published Japanese Patent Application No. 91,260,114 describes the heat treatment of fibers of a polymer of 70% 4-oxybenzoyl and 30% 6-oxy-2-naphthoyl monomer units at elevated temperatures (240.degree.-285.degree. C.) with a small amount of stress (2-6 grams per denier) to yield fibers with higher tensile strength and tensile modulus than fibers obtained by heat treatment of unstressed fibers. Published Japanese Patent Application No. 91,294,517 describes improvements in tensile properties of fibers made from the polymer of 73% 4-oxybenzoyl and 27% 6-oxy-2-naphthoyl monomer units; the property improvements are obtained by heat treating the melt spun fiber at 260.degree. C.-285.degree. C., followed by drawing of the fibers at elevated temperature (e.g. 280.degree. C). It appears that there are no reports of improvements in properties of heat treated thermotropic liquid crystalline polymers by applying a stress to the fiber at room temperature.